In a known type of nuclear reactor, the reactor core is of the heterogeneous type. That is, the core comprises a plurality of fuel assemblies or bundles, each vertically arranged in a spaced array to form the nuclear reactor core capable of self-sustained nuclear fission reaction. The core is contained in a pressure vessel wherein it is submersed in a working fluid, such as light water, which serves both as a coolant and as a neutron moderator. A plurality of control rods, containing neutron absorbing material, are selectively insertable among the fuel bundles to control the reactivity of the core. Such a reactor system is illustrated in greater detail, for example, in U.S. Pat. No. 3,382,153.
Each fuel bundle comprises a tubular flow channel containing an array of elongate, cladded fuel elements or rods supported between upper and lower tie plates. The fuel bundles are supported in the pressure vessel between an upper core grid and a lower core support plate. The lower tie plate of each fuel bundle is formed with a nose piece which fits through an aperture in the core support plate. The nose piece is formed with openings through which the pressurized coolant flows upward through the fuel bundle flow channels to remove heat from the fuel elements. A typical fuel bundle of this type is shown, for example, in U.S. Pat. No. 3,431,170.
Because of water gaps between fuel bundles, the distribution and position of the control rods, and other factors, the neutron flux and hence the power density is not uniform in a heterogeneous nuclear fuel core--not even within an individual fuel bundle. As a practical matter the power output of a nuclear reactor is limited by the fuel rod temperature limits at the peak power point in the core. To maximize the power output of the core (and of each fuel bundle) it is desirable to minimize the peak power-to-average power ratio, that is, it is desirable to "flatten" the power density variations. (This problem is discussed in greater detail, for example, in U.S. Pat. No. 3,147,191).
Several techniques have been proposed to accomplish power flattening within a fuel bundle. A well-known method is to appropriately vary the enrichment of the initial fuel in the fuel bundle. In practice this is accomplished by loading each fuel rod with fuel pellets of an enrichment appropriate to the position of the fuel rod in the fuel bundle.
The operation of known nuclear power reactors is based on the concept of an operating cycle. That is, reactor operation is periodically interrupted for refueling or reloading to restore the necessary reactivity. From the point of view of fueling or refueling the reactor core, the removable fuel bundle is the basic replaceable subdivision of the fuel core. According to known refueling schemes, only a fraction, for example 20-30 percent of the fuel bundles in the fuel core, are replaced at each refueling. Thus at any given time, the fuel core contains fuel bundles of various periods of fuel depletion. The degree of fuel depletion for each fuel bundle depends upon its residence time in the core. Thus, the required enrichment of the reload fuel depends not only on the anticipated position of the fuel bundle in the core, but also on the overall enrichment that must be added to the core to restore the desired amount of excess reactivity.
Ideally, the reload fuel would be designed on the basis of core operating data up to the time of the reactor refueling shutdown. As a practical matter this is not possible because of the lead time required for fuel bundle manufacture and because the refueling shutdown must be as short as possible. It is therefore highly desirable to reduce manufacturing lead time and to provide flexibility in the selection of fuel rod enrichment to provide a better match of the nuclear characteristics of the reload fuel with the requirements of the core.
In a known method of assembling fuel bundles, each rod is inserted by hand into a series of fuel rod spacers and a lower tie plate held on a support table. All the rods which are to go in a bundle are laid out side-by-side on a table in the order they are to be loaded. To assure against misplacement of fuel rods of high enrichment in the fuel bundle, means are provided to prevent the insertion of high enrichment fuel rods into low enrichment fuel rod positions of the fuel bundle. The shanks of the end plugs of high enrichment fuel rods are made of a larger diameter, as are the mating receptacle holes in the fuel bundle upper tie plate. This precludes insertion of high enrichment fuel rods into the smaller diameter receptacle holes of the low enrichment fuel rod positions.
A disadvantage of multiple diameter end plugs is that they require smaller coolant flow passages in the tie plates than would be necessary if all end plug shanks were of one, small diameter size.
The foregoing technique has been found to be an effective method of assuring accuracy in the assembly of fuel bundles. However, a significant amount of manual handling is involved with the possibility of worker radiation exposure, the possibility of damage to the rods and high labor costs. Additionally, when the group of rods which is inserted into a bundle is recorded, there is the possibility of transcription errors and the exact position of each rod may not be known.
The fixed drilling pattern of the receptacle holes in the fuel bundle tie plate has also been found to be a serious obstacle to the desired reduction in fuel bundle manufacturing lead time and to the desired flexibility to select particular fuel rod enrichments at the latest possible time, consistent with the availability of the assembled fuel bundles at the time of the reactor refueling shutdown. Additionally, specialization of parts increases production costs.